1. Field of the Invention
The present invention relates generally to prosthetic ankle implants. In particular, it is directed to a total ankle prosthesis device having a plurality of components that resemble the natural geometry of a human ankle.
2. Description of the Related Technology
For many years there has been considerable interest and activity with respect to ankle joint replacement, in which the degenerative articular surfaces are removed and replaced with an artificial joint called an ankle joint prosthesis, as a viable approach to the treatment of diseased or injured ankle joints. As the population ages, the demand for ankle joint prostheses is growing.
Fusion has long been an alternative to ankle arthroplasty. Fusion has its drawbacks. For example, there is a loss of motion in the ankle joint which may cause difficulties with other associated parts of the foot and leg. More recent research on the ankle joint has allowed for improved designs for ankle joint prostheses and better implant materials allowing ankle joint prostheses to dramatically improve in quality and longevity. Many types of ankle joint prostheses have been developed over the past thirty years.
U.S. Pat. No. 6,409,767 discloses an ankle joint prosthesis comprising a talus implant for implanting in or on the talus and a top element including a tibia implant for implanting in or on the base of the tibia. The top element and the talus implant are mounted to move relative to each other by friction on a contact interface so as to allow the ankle to move. The contact interface presents a friction surface that can be considered a portion of a substantially frustoconical surface. When implanted, the substantially frustoconical surface is oriented so that its larger radius portion is directed substantially towards the lateral side of the ankle in accordance with the postulate of Inman's Joints. The top surface of the talar implant has two ribs on both edges running from the anterior to the posterior edges.
One example of an ankle joint prosthesis is disclosed in U.S. Pat. No. 7,025,790, which describes an ankle joint prosthesis comprising tibial, talar and mobile or semi-constrained bearing components that may be implanted in a patient. The top surface of the tibial component has a convex curvature and is configured so as to approximate and match with the curvature of a prepared portion of the distal tibia. The bottom surface of the tibial component is approximately flat. The top surface of the talar component has a saddle-shaped, convex curvature in its anterior to posterior plane. The bottom surface of the talar component has a concave curvature and is configured so as to approximate and match with the curvature of a prepared portion of the talus.
Inman's Joints of the Ankle, second edition, Chapter 2, “Functional Morphology of the Trochlea,” pp. 7-13, Ed. James B. Stiehl, M.D., Williams and Wilkins (1991) (hereinafter “Inman's Joints”), postulates that the trochlea of the talus is a section of a frustrum of a cone whose apex is directed medially and whose apical angle varies considerably from individual to individual. Many of the prior art devices are based on this postulate. However, the present device is based on a different postulate, namely that the talus is a section of a frustrum of a cone whose apex is directed substantially in a lateral direction rather than in a medial direction. This type of device is characterized by a medial curvature larger than the lateral curvature as viewed in sagittal plane cross-sections.
WO 2006/023824 discloses an ankle joint prosthesis including a talar component having a lower surface with a bone fixation portion for fixation to the talus and an upper surface designed for articulation using a bearing component. The bearing component can have a lower surface for articulation relative to the talar component and an upper surface for articulation relative to the tibial component. The tibial component can have a lower surface for articulation relative to the bearing component and an upper surface with a bone fixation portion for fixation to the tibia and/or fibula. The talar component has height greater on the lateral side than on the medial side.
U.S. Pat. No. 7,615,082 discloses an artificial joint, particularly for replacing a talocrural joint, including a first primary joint surface that forms an articular fossa for replacing the tibia and having concave curvature extending parallel to a primary functional plane of the joint, which corresponds to the sagittal plane, and a second primary joint surface which cooperates with the first primary joint surface as a component of a condoyle that replaces the talus and has convex curvature on the primary functional plane that is adapted to the first primary joint surface. The radius of the second primary joint surface decreases from the lateral side to the medial side of the joint in accordance with the postulate of Inman's Joints.
After initial encouraging results, follow-up clinical studies on many of these ankle joint prostheses revealed frequent failures of such implants due mainly to the inadequate restoration of the natural mobility and the poor stability of the resulting ankle implants. Many of the problems originated from our poor understanding of the relative contribution of the ligamentous structures and functional morphology of the articular surfaces in providing passive and active stability for the human ankle joint.
One objective of the present invention is to provide an improved prosthetic ankle that more closely approximates the natural geometry of the articulating bones in the ankle joint. The ankle prosthesis will allow mobility characteristics that more closely resemble the movement of a natural ankle than many prior art devices.